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Hitting the gym

Some behaviors that in moderate amounts keep you healthy can actually weaken your immune system when taken to the extreme. For example, overexercising can leave your body struggling to cope with added physical stress—especially if you're not sleeping, hydrating, and fueling your body adequately. Unfortunately, the gym is also a great place to pick up viruses, from the sweaty treadmill to the benches in the locker room; plus, germs likely even catch a ride home on your gym bag.

We're not saying you should skip out on exercise: When done right, it will keep your immune system strong against the flu. But to protect yourself, wipe down machines before using them and take your own mat for stretching—or cover a borrowed one with a towel. Shower with soap and water immediately after working out to help kill any germs you may have been exposed to.

Next: Drinking alcohol